Channel catfish (Ictalurus punctatus) are widely cultured because they are easily managed and have good food quality. Although much research has been performed on rearing, stocking, and nutrition of channel catfish, little research has been directed toward genetic improvements that would increase its value as a cultured species.
One genetic improvement that provides benefits to fish culture is the production of triploid (sterile) individuals. Triploidy is one type of polyploidy which may be generally described as a specimen with three complete sets of chromosomes in place of the usual two sets. Natural spontaneous triploidy in fish is rare. However, triploidy has been induced in certain fish species by cold-shocking fertilized eggs. For example, in the threespine stickleback Gasterosteus aculeatus as described by H. Swarup in "Production of triploidy in Gasterosteus aculeatus (L)," Journal of Genetics, 56:129-142, 1959a, cold-shocking was used for the purpose of inducing triploidy. However, using the suggested temperature and times the highest percentage of triploidy induced was 66.7%.
Again, respecting plaice and hybrid flounders a study was made and reported by C. E. Purdom in "Induced Polyploidy in plaice (Pleuronectes platessa) and its hybrid with the flounder (Plathichthys flesus)," Heredity, 49:11-24, 1975, using cold-shocking techniques to induce polyploidy. This study used different times and temperatures for application of the cold-shocking techniques and diploids, triploids, and even tetraploids were obtained. However, it was reported at page 16 of the article that "Survival percentages were similar in all groups up to 12 days after fertilisation, except for the triploid hybrids (italics supplied);this group of eggs showed catastrophic death rate for which no explanation could be found." However, after metamorphosis, it was reported that viability of triploids was as good for diploids produced by the cold-shock treatment. This study also claimed that of the fertilized eggs of the hybrid flounders treated that survived the cold-shock to hatching, all appeared to be triploids. No chromosome check was made but some data was presented which was claimed by Purdom to verify his conclusion.
Also, respecting the blue tilapia, cold-shocking was reported for the purpose of the induction of polyploidy by R. J. Valenti, "Induced polyploidy in Tilapia aurea (Steindachner) by means of temperature shock treatment," J. Fish. Biol., 7:519-528, 1975. The results reported suggested that a cold shock at about 4.degree. C. had a low percentage of hatch. After hatch, the mortality was described as low. It was recognized that using different temperature shocks as well as different durations of exposure, is highly significant. But no shock temperatures and exposure durations were disclosed which obtained consistent, or even verifiable by chromosome count (which was not done), obtention of triploidy fish.
Even though it would appear that studies done with these and other species would suggest the present invention, the fact is that applicants are the first to devise a method for production of 100% triploid channel catfish.